The Mouse Brainstem (Truncus encephali)

The mammalian brainstem is composed in caudocranial order of the medulla oblongata, the pons, and the midbrain (mesencephalon). The main difference to the spinal cord (see ► Chap. 5, nervous supply of trunk and limbs) is the endowment of the brainstem with nuclei, fiber tracts, and nerves for the innervation of the head-specific organs, like the eyes, the ears, the organ of equilibrium, the nose, the foregut, and the specific muscles of the head and face region. The nerves providing eye movement (oculomotor, trochlear, and abducens), the face (trigeminal and facial), the inner ear (vestibulocochlear), and the foregut (trigeminal, facial, glossopharyngeal, vagal and hypoglossal) originate (motor, efferent innervation) or end (sensory, afferent innervation) in specific nuclei inside the brainstem. Following a historical classification, there are nine “real” cranial nerves, comparable to the spinal nerves of the spinal cord, and two cranial nerves which in effect are bulges of the telencephalon (olfactory nerve, olfaction; see ► Chap. 14) and the diencephalon (optic nerve, vision; see ► Chap. 8). The accessory nerve, innervating some of the neck muscles, is included additionally into the cranial nerves although its neurons of origin are located in the cervical spinal cord.